This invention relates to improvements in an AC elevator control system.
There are known AC elevator control systems of the type comprising a converter for converting a three-phase electric power to a direct current, an inverter including a plurality of inverting elements to invert the direct current from the converter to a three-phase electric power with a variable voltage at a variable frequency and a three-phase induction motor receiving the three-phase electric power from the inverter to drive an associated elevator car with a rotational speed thereof controlled. Upon the occurrence of a power failure or a fault, a battery connected across the DC side of the inverter takes over the direct current from the converter and the induction motor is put in the regenerative mode of operation. Thus a regenerative electric power is produced on the DC side of the inverter. In order to return that regenerative power back to the AC side of the converter a regenerative inverter has been connected across the converter.
When a voltage on the DC side of the inverter rises in the regenerative mode of operation, selected ones of the inverting elements of the inverter have been switched to supply a direct current to the induction motor. Thus a DC braking action is exerted on the motor to cause the elevator car to travel at a constant speed. Under these circumstances the induction motor has been rotated at such a rotational speed that a braking torque of the induction motor is balanced with an unbalanced load torque resulting from a difference in weight between the elevator car and a counter weight connected the car through a traction rope. That rotational speed has been unstable and therefore, in order to stabilize the rotational speed, an emergency speed instructing device has compared a command emergency speed signal from a command emergency speed generator with an actual speed signal for the elevator car delivered from a tachometer generator connected to the induction motor to supply a command speed signal to the inverter to effect the feedback control of the motor's speed.
Therefore the abovementioned type of conventional AC elevator control systems has been disadvantageous in that the resulting apparatus is not only expensive but also an operating device enabled upon the occurrence of an emergency is complicated and the reliability thereof is not sufficiently satisfied.
Accordingly, it is an object of the present invention to provide a new and improved AC elevator control system ensuring that an associated elevator car can be operated with a cheap structure and through the simple control upon the occurrence of a power failure or a fault by controlling an inverter involved to generate a low frequency to lower a heavier one of the elevator car and an associated counter weight at a low speed.